a) Field of the Invention
The present invention relates to a filed emission element and more particularly to a field emission element having a field emission cathode whose tip emits electrons and its manufacture method.
b) Description of the Related Art
A field emission element emits electrons from a sharp tip of an emitter (field emission cathode) by utilizing electric field concentration. For example, a flat panel display can be structured by using a field emitter array (FEA) having a number of emitters disposed on a substrate. Each emitter controls the luminance of a corresponding pixel of the display.
FIGS. 15A to 15F schematically illustrate a conventional method of manufacturing a field emission element.
As shown in FIG. 15A, a conductive gate electrode 62 is formed on a substrate 61. For example, the conductive gate electrode 62 is made of polysilicon doped with impurities. On the conductive gate electrode 62, a resist film 63 having a predetermined pattern is formed through photolithography.
Next, by using the resist pattern 63 as a mask, the gate electrode 62 is anisotropically etched to leave as shown in FIG. 15B a gate electrode 62a having a gate hole 67 having a circular flat shape (as viewed from the top). This etching thins the resist pattern 63 and a thin resist pattern 63a is left.
As shown in FIG. 15C, after the resist pattern 63a is removed, a sacrificial film 64 is isotropically deposited on the gate electrode 62a and on the exposed substrate 61.
Next, as shown in FIG. 15D, the sacrificial film 64 is anisotropically etched to leave a sacrificial film (side spacer) 64a on the side wall of the gate hole 67 of the gate electrode 62a.
Next, as shown in FIG. 15E, an insulating film 65 is formed on the whole upper surface of the substrate and a conductive emitter electrode 66 is formed on the insulating film 65.
Next, as shown in FIG. 15F, the whole of the substrate 61 and side spacer 64a and part of the insulating film 65 are etched to leave a peripheral portion of the insulating film 65a between the gate electrode 62a and emitter electrode 66.
As a positive potential is applied to the gate electrode to concentrate an electric field upon the tip of the emitter electrode (cathode) 66, electrons can be emitted from the emitter electrode 66 toward an anode electrode (not shown).
FIG. 16 is a cross sectional view of a flat panel display using such field emission elements.
Each field emission element is manufactured by the above-described method, and has an emitter electrode 44 and a gate electrode 45. Formed on a support substrate 41 made of insulating material are a wiring layer 42 made of Al, Cu, or the like and a resistor layer 43 made of polysilicon or the like. On the resistor layer 43, a number of emitter electrodes 44 having a sharp tip are disposed to form a field emitter array (FEA). Each gate electrode 45 has a small opening (gate hole) near at the tip of each emitter electrode 44 and a voltage can be applied independently to each gate electrode although not specifically shown in FIG. 16. A plurality of emitter electrodes 44 can also be independently applied with a voltage.
Facing an electron source including the emitter electrode 44 and gate electrode 45, an opposing substrate is disposed including a transparent substrate 46 made of glass, quartz, or the like. The opposing substrate has a transparent electrode (anode electrode) 47 made of ITO or the like disposed under the transparent electrode 46 and a fluorescent member 48 disposed under the transparent electrode 47.
The electron source and opposing substrate are joined together via a spacer 50 made of a glass substrate and coated with adhesive, with the distance between the transparent electrode 47 and emitter electrode 44 being maintained about 0.1 to 5 mm. The adhesive may be low melting point glass.
Instead of the spacer 50 of a glass substrate, a spacer 50 made of adhesive such as epoxy resin with glass beads being dispersed therein may be used.
An air exhaust pipe 49 is coupled in advance to the opposing substrate. By using this air exhaust pipe 49, the inside of the flat display panel is evacuated to about 1.times.10.sup.-5 Torr to 1.times.10.sup.-9 Torr (about 1.times.10.sup.-5.times.133.3 Pa to 1.times.10.sup.-9.times.133.3 Pa), and then the air exhaust pipe 49 is sealed by using a burner or the like. Thereafter, the anode electrode (transparent electrode) 47, emitter electrode 44, gate electrode 45 are wired to complete the flat panel display.
The anode electrode (transparent electrode) 47 is always maintained at a positive potential. Pixels are selected two-dimensionally by emitter wiring lines and gate wiring lines. Field emission elements are selected disposed at each cross point of voltage applied emitter and gate wiring lines.
The emitter electrode is applied with a negative potential and the gate electrode is applied with a positive potential. Electrons are emitted from the emitter electrode toward the anode electrode. When electrons are bombarded with the fluorescent member 48, fluorescence is radiated from the bombarded area (pixel).
In order to maintain the inside of the flat panel display at a high vacuum degree, a getter member 51 is provided at the corner in the flat panel display. For example, the getter member 51 is made of Ti, Ta, Zr, Al, Mg, or the like. After the air exhaust pipe 49 is sealed, the getter member 51 is activated by heating it with a lamp or laser beam to adsorb ambient molecules therein. The initial vacuum degree in the flat panel display can therefore be improved.
Other molecules such as He passing through the transparent substrate 46 and support substrate 41 and entering the inside of the flat panel display or other molecules such as H.sub.2 O, O.sub.2, and N.sub.2 emitted in the flat panel display are also adsorbed by the getter member 51. As a result, the vacuum degree in the flat panel display is prevented from being lowered and the flat panel display is prolonged its lifetime.
The getter member 51 is disposed at the corner in the flat panel display so as not to obstruct electrons to be emitted from the emitter electrode 44 toward the transparent electrode 47. The getter member 51 is therefore placed at the position remote from the emitter electrodes 44. As the getter member 51 is placed remotely from the emitter electrodes 44, the function of the getter member 51 cannot be sufficiently demonstrated and the following disadvantages may occur.
(1) Molecules described above are attached to the surface of the emitter electrode 44 in a high electric field, before they are adsorbed by the getter member 51. Radiation current (electron flow) from the emitter electrode 44 therefore reduces.
(2) As molecules attach to or emit from the surface of the emitter electrode 44, a magnitude of the radiation current from the emitter electrode 44 fluctuates and becomes unstable.
(3) As the emitter electrode 44 is bombarded with ions, the emitter electrode 44 is sputtered and the tip of the emitter electrode 44 deforms. As the tip of the emitter electrode 44 is rounded, an electric field is hard to be concentrated and the performance of the field emission element is degraded.
(4) In order to maintain a high vacuum degree during the long span of one to ten years, a getter member 51 having a large area is required. As the large area getter member 51 is used, the flat panel display becomes large and the fluorescence radiation area (display area) becomes relatively small.